Plastic materials are broadly divided into two main classes based on the material's response to heat, thermoplastics and thermosetting resins. Thermoplastic resins, when heated, soften or melt and flow as liquids and when cooled, they solidify. These changes on heating and cooling can be repeated several times without appreciable degradation. Virtually all thermoplastic products are made by melting thermoplastic compounds, shaping the molten plastic, and cooling it while maintaining the shape. In contrast, thermosetting resins are cured, hardened, or “set” into a permanent shape. Curing is an irreversible process whereby permanent cross-linking occurs. Thus once molded, a thermoset product cannot be reheated and molded again.
The majority of resins produced are thermoplastics. Although a number of chemically different kinds of thermoplastics are commercially available, they can be divided into two broad classes: amorphous and crystalline. The latter are characterized by melting and freezing points. Amorphous resins do not have melting points, but rather are defined by a glass transition temperature, Tg. Common amorphous thermoplastics are polystyrene, polycarbonates, poly(methyl methacrylate), and poly(vinyl chloride). Crystalline or partially crystalline thermoplastics are often described by melting temperature and a glass transition temperature. These materials are processed above their melting points and then cooled to cause crystalline domains to form. Examples would include polyethylene, polypropylene, polyethylene terephthalate, and Nylon.
Thermoplastic elastomers are block copolymers having one or more alkenylarene polymer blocks and one or more olefinic polymer blocks. The block copolymers are elastomeric in the sense that they typically have a three-dimensional, entangled (alternatively known as “physically crosslinked”) structure below the glass transition temperature (Tg) of the alkenylarene block such that they exhibit elastic memories in response to external forces. The block copolymers are thermoplastic in the sense that they can be softened or melted above the glass or crystalline transition temperature of the alkenylarene block, processed, and cooled/solidified several times with little or no change in physical properties (assuming a minimum of oxidative degradation).
Thermoplastics are fabricated into useful shapes and articles using thermal and mechanical processes to manufacture plastic products, articles, films, fibers, or coatings, for example. A particular polymer is usually chosen based on the mechanical, thermal, or visual properties desired in the product. For a particular fabrication process, e.g., extrusion, film blowing, molding, coating, and forming, the process variables (temperatures, flowrates, pressures, scrap, cost, etc.) are typically selected such that the rheological properties of a polymer are adequate. However, the selections of polymer and process are highly related. In certain situations, the polymer may degrade at elevated temperatures or the equipment reliability is unacceptable at high temperatures. Additionally, the theological properties of some polymers may render certain processes or process conditions impossible or non-optimal, which may preclude these polymers from being employed regardless of their mechanical properties.
Plasticizers or processing oils are often added to block copolymers to lower the viscosity and improve the processability of block copolymers. Other polymers may also be added to compatibilize the blends and/or improve the mechanical properties. Blends comprising block copolymers are reportedly described in U.S. Pat. Nos. 3,562,356; 4,704,110; 4,578,302; 5,503,919; 5,540,983; 6,117,176; and 6,187,425. However, the addition of plasticizers and/or processing oils lowers the mechanical properties of the block copolymer compositions.
U.S. Pat. No. 4,131,581 to Coker relates to crystalline solvents for a polymer component that must be miscible with a viscosity reducing diluent. U.S. Pat. No. 5,945,485 to Struglinski et al. relates to viscosity modifier polybutadiene polymers, and U.S. Pat. No. 5,633,319 to Silvi et al. relates to compatibilized blends of polyetherimides and liquid crystalline polyesters, optionally with a minor proportion of a non-liquid crystalline polyester.
The present inventors provide herein phase change solvents that are able to reduce viscosity of a thermoplastic at high temperatures for processing while not substantially compromising the mechanical strength of the polymer at use temperatures.